Electrolytic process for producing metals



June 6, 1961 B. KOPELMAN ETAL 2,987,454

ELECTROLYTIC PROCESS FOR PRODUCING METALS Filed Nov. 2?, 1957 INVENTORS BERNARD KOPELMAN 8 ROBERT B. HOLDEN flan d fl- United States Patent 2,987,454 ELECTROLYTIC PROCESS FOR PRODUCING METALS The present invention relates to a method for producing metal articles from difiicultly reducible compounds thereof by electrolysis.

In recent years, a great need has developed for large quantities of metals which have hitherto not been employed extensively in the metallurgical field. For example, a large demand has developed in the atomic enrergy field for beryllium, uranium, zirconium and other metals having special nuclear properties.

It is accordingly one of the objects of the subject invention to provide a novel and economic process for the production of elemental uranium and other metals which are normally combined in the form of difiicultly reducible compounds.

Another object of the invention is to provide metals directly in a finely divided state suitable for powder metallurgical conversion to various forms without the necessity for first finely dividing the metal preparatory to powder metallurgical processing.

Still another object of the present invention is to provide a scheme for preparing powder metal compacts of difficultly reducible and pyrophoric metal specimens without exposing said metals to air or other gases. Other objects will be in part apparent and in part pointed out hereinafter.

In one of its broader aspects the objects of the invention are achieved by providing a fused bath containing the compound to be reduced electrolytically and at least one salt of the alkali and alkaline earth metals, heating said bath to a temperature above its melting point and below the boiling point of mercury, electrolyzing said bath while employing mercury as a cathode and recovering metal from the mercury of said cathode.

It has been found that a composition results from this electrolysis which can be used to prepare powder metallurgical specimens directly. Such preparation is accomplished by introducing the composition containing mercury and the electrodeposited metal, directly into a die, pressing and heating the composition to drive off the mercury and to sinter and compact the metal residue in the die. The scope of the present invention includes the complete conversion of the difiicultly reducible metal compound to the metal compact with a minimum of handling and without removal from the mercury until the compact is in the final stages of formation.

In one of its preferred embodiments the invention involves the electrolysis from a mixture of the halide of the metal to be reduced and a eutectic mixture or nearly eutectic mixture of alkaline halides and alkaline earth halides. The halides are preferred because of their stability, low melting points, availability and the good results obtained through their use.

The mercury cathode of the electrolytic cell is preferably located at the bottom of the cell. This is preferred because of the ease of addition and removal of the mercury from this portion of the bath and because of the large surface area which it presents to the solution. Also because of its high density the mercury tends to be found at the lowest portion of the cell and no special containing means are needed.

The mass of salt employed in the bath to be electrolyzed serves as a solvent for the compound to be reduced and should preferably be molten at temperatures below the boiling point of mercury. This is preferred in order to avoid high pressure operations which are made necessary by the use of higher temperatures, i.e. above the boiling point of mercury. The boiling of mercury is preferably avoided because of its toxic properties and handling difficulties. However where the circumstances dictate the use of mercury above its boiling point, it may be carried out where super atmospheric pressures are also employed. Such circumstances are, for example, the desirability of having higher concentrations of the compound in the bath where its solubility is low at temperatures below the boiling point of mercury.

It is noteworthy in this regard that even a very slight solubility of the compound in the salt bath is sufiicient to enable the process to be carried out. It is not necessary that the compound dissolve completely in the salt solvent. However it is preferred to maintain an appreciable concentration of the compound therein and this may be done by dispersing the undissolved solid by proper choice of salt solvents, or by other conventional techniques.

The accompanying drawing is a diagrammatic view, in elevation and partly in section, of an electrolytic appar-atus in which the method of the present invention may be carried out.

Mercury boils at 357 C. at atmospheric pressure and as noted, it is preferred to carry out the electrolysis below this temperature. The metal which is formed electrolytically deposits in the mercury of the cathode to form a so-called quasi-amalgam. The process may be carried out batchwise. In this case, after the electrolysis is completed and the ceil has cooled down, it will be noted that the quasi-amalgam containing the electrolyzed metal product is semi-solid or mushy liquid and can be readily separated from the frozen salts. It is particularly noteworthy that the product of the electrolysis is elemental meta and not simply a lower valence product such as a lower oxide.

One distinct advantage of the method as employed in connection with powder metallurgy techniques is that the metal formed by the electrolysis is found in the mercury in finely divided form. The powder is in fact more finely divided than that produced by other electrolytic methods and is in the order of 5 in size. By previously known methods particles in a 20 to p size range were produced. This method avoids the diificulty which is found to be a drawback in the electrolytic processes which do not employ the mercury cathode because very finely divided powder particles are produced in the form and condition suitable for hot pressing directly into powder metallurgical compacts. The metal produced by conventional electrolytic procedures, and without the mercury cathode, must be separated from the salt and leached to remove traces of salt and it is therefore necessary to produce such metal in such large crystal form in order that it may withstand the leaching. The leaching of the large crystals represents a considerable loss of metal which must then be reprocessed. In addition the present process is carried out at notably lower temperatures than those which do not employ the mercury cathode.

In hot pressing the mercury evaporates and forms a dense protective atmosphere around the metal powder. This effectively prevents oxidation or other contamination and insures a pressed slug of the highest density. In some instances, particularly when the metal powder is pyrophoric or combines with or dissolves relatively inert gases such as nitrogen or hydrogen, this has a distinct advantage over pressing powder in other gas atmospheres because the mercury atmosphere is much more dense and therefore more protective than hydrogen or nitrogen or similar atmospheres. Moreover the atmosphere forms in the compact and spreads out from it. Thus from the time the metal is formed in the mercury until the time it is formed into the metal compact it is protectively enveloped in mercury.

Numerous metals may be reduced from the difficultly reducible compounds to the native metal by this method. Among the metals which may be reduced are: uranium, beryllium, titanium, zirconium, thorium, chromium, manganese, silicon, vanadium, magnesium, aluminum and the rare earths. It is applicable to any metal oxide which is at least as stable as chromium oxide. Among'the metal compounds which may be reduced are fluorides chlorides and oxides. Examples of these compounds are UF UCl K UF U and other uranium oxides, beryllum oxides, beryllium chloride, beryllium fluoride, and the oxides or halides of thorium, zirconium and titanium.

The metal halides to be electrolyzed should, as noted above, have at least some solubility in the salt bath composition. Solubilities as low as 1% are sufficient and permit the electrolysis to run continuously. In this connection one distinct advantage of the method is that the mercury may be drained from and introduced into the electrolytic bath in a continuous manner in order to remove electrodeposited metal continuously from the bath. The electrolyzed compound can similarly be introduc ed into the electrolytic bath continuously to provide a very economical continuous processing.

The salt composition which serves as the solvent for the compound to be electrolyzed is selected from the group of alkali salts and alkaline earth salts, and a eutectic composition of a composition having nearly the eutectic proportions is preferably used in order to provide a low melting point, i.e. below the boiling point of mercury.

The process does not lead to the formation of a true amalgam. The resulting composition is however in suitable form for direct compacting and sintering according to powder metallurgical techniques to yield a formed metallic shape. The product of the electrolysis appears to be a fine dispersion of metal in mercury to which the term quasi-amalgam might be applied. The method may also be employed in forming a very finely dispersed hydride of the metals which form stable hydrides. This modification of the process is carried out by combining hydrogen gas with the metal as it forms. The following is an example of one procedure which may be followed in carrying out this modification of the procedure.

Example I Approximately 80 grams of LiBr, 73.9 grams of KBr and 26.1 grams of CaBr were combined to make up a eutectic bath containing approximately 180 grams of salt. Twenty grams of UCl were added to the bath. The bath was electrolyzed employing a mercury cathode and employing a current of 3 to 5 amperes for about 2 /2 hours at a temperature of about 339 C. The mercury cathode covered the bottom of the electrolytic cell and had a depth of 8.2 centimeters. Hydrogen was passed through the mercury and salt during the electrolysis. A semi-solid uranium hydride quasi-amalgam was obtained.

Numerous salts and combinations of salts selected from the group consisting of alkali and alkaline earth metals may be employed; for example, aluminum and sodium chlorides, the sulfides of cesium or potassium, acetates of magnesium, barium nitrate, aluminum fluoro-sulfate and potassium hyposulphite.

With reference to the attached figure, an electrolytic cell composed of type 304 stainless steel had a 10-inch diameterand a 14-inch height. Two stainless steel tubes 12 and 16 were welded into the sides of the cell at the base. The tube 1 6 having a inch diameter was the mercury return line from the mercury circulation system described below. Tube 12 having a 1-inch diameter served as the amalgam exit. A water-jacketed condenser 14 on tube 12 served to cool the hot amalgam from the cell to room temperature. A 9-inch diameter Pyrex battery jar 20 with the base cut off was used for the feed container sleeve. This sleeve prevented direct contact of the salt 18 with the sides of cell 10. A rectangular piece was cut out of the base of the sleeve to uncover the opening to the exit tube 1'2 and permit amalgam to flow out of the cell. The cell top 22 limited vertical move ment of the sleeve.

A bed of mercury 24 at the bottom of cell 10 served as a cathode. The mercury was stirred by a stainless steel stirrer having blades 26, a shaft 23 and conventional mercury seal 30 provided in the cell top 22. A glass sleeve 32. protected the rod from the molten salt. A stainless steel pin 33 prevented vertical movement of sleeve 32. The lower tip of shaft 28 terminated in a recess 34 in the base of cell 10 and prevented wobbling.

An anode 36 was a %-inch diameter rod of calcined carbon black. A stainless steel rod 38 was threaded into the carbon anode and, being ofiset, considerable lateral movement of the anode was permitted without shorting the cell when this occurred.

Heat was supplied to the cell by means of electrically heating nichrome ribbon winding 40 and by a hot plate (not shown) beneath the cell. The cell sat in a low melting metal bath 42 within a container 44 on the hot plate. Winding 40 was heavily insulated with asbestos.

The upper part of the cell, i.e. above the bath 42, was not insulated and thus ran much cooler than the heated part. It acted as an efiicient mercury condenser and also permitted the use of rubber stoppers 46 and 48 for making the top seals. The cell top 22 was bolted to the base 10 by bolts not shown; a seal being effected by a metal O-ring 50.

Feed salt was added to the cell through an opening 52 in the cell top 22 covered by a pinched off piece of tubing 54. The temperature bath was controlled by conventional thermostatic means. Thermocouples were maintained at the bottom of the cell, in the mercury bath, and in the center of the salt layer. Salt depth measurements could also be made through opening 52. A positive argon or hydrogen atmosphere was maintained in the cell, these and other gases being introduced and removed through ports not shown in cover 22.

The mercury circulation system served to remove amalgam from the cell continuously, to separate the beryllium from the majority of the mercury, and to return filtered mercury back to the cell 10. Mercury passing from condenser 14 through tube 12 passed to a glass viewer 60. Two-way stopcock 62 permitted introduction of argon under pressure into, or evacuation of gas from viewer 60 through the lines indicated. It also permitted evacuation of individual separators 64 when they were placed in the system. The viewer 60 provided a means for checking the mercury level visually and also gave an indication of amalgam both quantitatively and qualitatively. Where the atmosphere within the cell is well controlled, this viewer is not necessary. As used, the viewer was about 4 inches in diameter and 5 inches high. V

The separators 64 used were 3.5 inches-in diameter and 4.5 inches high and were made of mild steel. 7 A 200 mesh stainless steel screen backed by a 20 mesh screen 66 served to retain the beryllium amalgam in the separator while allowing the mercury to pass through. A rubber gasket 68 sealed the separator interior from the atmosphere at the external opposing faces of the top section of the separator and the screw-on type base where they were joined at the threaded section 70. The screen was compressed into place in the separator at its edges by the internal opposing faces of the top and base of the separator. A mercury storage reservoir 72 may take any convenient form and in this case was simply a glass separatory funnel. It is opened to the system by valve 74 only when additional mercury was needed. A mercury pump 77 transferred mercury from the separator 64 through tubing 76 to preheater 78. The mercury preheater was a Woods metal bath in a container 80. The bath was electrically heated with a resistance winding 82 as well as internally heated with a 2500 watt quartz immersion heater (not shown). The temperatures of the bath served as control for the heaters.

The use of hydrogen gas in the cell was found to be unnecessary where the entire cell system is scrupulously gas tight. The use of hydrogen or hydrogen-argon mixtures may in some cases be advantageous. Such mixtures when bubbled through the mercury cathode promote amalgam formation. Argon with a dew point of lower than 60 was employed. Current efficiencies were measured by analysis of chlorine gas mixed with argon gas which passed from the cell 10.

Pure BeCl feed in an argon filled container was introduced to cell 10 as needed through tube 54 by opening pinch clamp 55. Introduction through a T connection which could be evacuated prevented contact of BeCl with the atmosphere. BeCl and beryllium amalgam are both extremely sensitive to air and moisture and every precaution was taken to prevent access of the latter to the system.

At startup, distilled mercury was introduced into the air-tight system in sufiicient quantity to fill the circulation system and to fill the cell 10 to a point about /2 inch above the rectangular cut in glass sleeve 20. Tygon tubing has been found appropriate in connecting sections of the apparatus as for example at 90. The original feed to the cell consisted of approximately 13.5 pounds of BeCl and 6.5 pounds of NaCl (a 60:40 molar ratio). Since essentially no NaCl is lost during operation the salt depth is a fair measure of the BeCl concentration. The mercury height above the opening in the sleeve 20 is adjusted and maintained at a point A to inch above the rectangular sleeve outlet using mercury from reservoir 72. Greater height should be avoided as it may lead to difficulty in getting the amalgam, which is less dense than mercury, out of the cell and into the side arm. If the mercury level is set too low, salt may get into the side arm and freeze in the condenser. The anode was positioned at a height of about /1 inch above the cathode surface.

The cell was brought up to operating temperature and the circulation pump 77 set to a predetermined delivery speed in the range 1.3 to 3 gallons per hour and the stirrer set to the desired speed in the range of 50 to 125 revolutions per minute. Excess mercury was recirculated by pump 77 through by-pass conduit 90. About 1 /2 liters per minute of argon was flowed through the top of cell 10. Electrolysis was then started and the current set to between 30 and 60 amperes. A voltage of between 6.5 and 8.3 volts was employed. Salt was maintained at a temperature between 308 and 326 C. The mercury depth was about 1.5 inches, the salt depth about 4.25 inches. A stirring rate of between 50 and 100 revolutions per minute was used. A current efliciency of 87.1% was achieved. When the separator had accumulated sufficient amalgam, tygon tubing section 13 and 15 were pinched off and the separator removed. A second separator was put in its place, the system evacuated through stopcock 62, and the tubing 13 and 15 reopened to resume operation after addition of mercury from reservoir 72. Beryllium chloride was added to the cell as needed. An operating range of 45 to 60% BeCl or 7.3 to 13.5 pounds of BeCl was maintained in cell 10. A 2400 ampere-hour run at 80% current efliciency was possible before recharging was necessary. To minimize concentration effects, additions were made at about 600 ampere-hour intervals. Current efliciencies were subject to variation of stirrer speed. An efiiciency of about 80% was obtained at the stirrer speed of about 100 revolutions per minute. Increase in amperage resulted in decrease in efliciency,

The amalgam produced by this electrolytic process can be diluted with mercury in all proportions and on stirring becomes essentially homogeneously dispersed. On standing a relatively thick amalgam forms as an upper layer leaving essentially pure mercury below, thus permitting easy separation. The amalgam is changed in form on contact with air to a black powder and accordingly should be protected from atmospheric contamination. Pure tank nitrogen is effective for this purpose. Vacuum filtration of the amalgam produces a semi-solid cake having a shiny and silvery metallic appearance. This cake contains 1.7 to 2.0% beryllium by Weight and corresponds to Be(Hg) with a slight excess of mercury. Since the viscosity of the amalgam increases with increasing beryllium content the mercury circulation in the cell system must be sufiicient to prevent plugging by amalgam. Maintaining beryllium concentration below 0.25 weight percent prevents such plugging and this amalgam may be flowed through a capillary of one millimeter radius. Oxygen content of the amalgam produced in this experimental apparatus was less than 0.7% and chlorine less than 0.3%. The amalgam from which excess mercury has been removed by vacuum filtration may be hot pressed directly into beryllium compacts. Hot pressing in a die lubricated by Aquadag gave a product the center of which contained about 0.1% carbon and 0.75% oxygen. The hot pressing conditions used were generally about 3 to 4 minutes at 1000 C. and 1000 p.s.i. and then 15 minutes at 900 C. and 4000 p.s.i. From a number of such pressings it was demonstrated that continuous pressure applied during heating produced compacts with low oxygen content, 0.7 to 0.8%, but relatively high chlorine content, 0.1 to 0.3%, while pressing only when the pressing temperature had been reached produced compacts with low chlorine, 0.00 to 0.07% but relatively high oxygen content, 1 to 7%. The longer the time at pressing temperature the higher the oxygen content and the lower the chlorine content. The compacts obtained had grain sizes of less than 50 microns. No residual mercury was detected.

As an alternative the beryllium powder may be produced from the amalgam by low temperature vacuum distillation of the mercury. A temperature of 340 C. may be used for the purpose. Some BeCl contaminant is also removed in this manner. The residual beryllium was loosely sintered and could easily be ground to a fine powder. Close control of atmosphere and minimum processing time should be employed. Further chlorine removal results from high temperature operations such as hot pressing or vacuum sintering.

Since many embodiments might be made in the present invention and since many changes might be made in the embodiment described, it is to be understood that the foregoing description is to be interpreted as illustrative only and not in a limiting sense.

We claim:

1. The method of reducing the halide of beryllium comprising the steps of incorporating said halide in a eutectic bath containing as its essential component at least one of the alkali and alkaline earth metal halides, said bath having a melting point less than the boiling point of mercury, electrolyzing said bath while employing mercury metal as the cathode to deposit the metal of said halide in said mercury metal and separately collecting mercury containing the metal of said halide and thereafter separating the metal of said halide from mercury.

2. The method of reducing the halide of beryllium comprising the steps of incorporating said halide in a eutectic bath containing as its essential components beryllium chloride and sodium chloride, electrolyzing said bath while employing mercury as the cathode to deposit the metal of said halide in said mercury metal and separately collecting mercury containing the metal of said halide and thereafter separating the metal of said halide from mercury.

3. The method of reducingberyllium halide compris' ing the steps of incorporating said halide in a bath containing as its essential component at least one of the alkali 'as the cathode to deposit beryllium in said mercury metal and separately collecting mercury containing beryllium and thereafter separating the metal of said halide from mercury.

4. The method of reducing beryllium halide comprising the steps of incorporating said halide in a bath containing beryllium chloride and sodium chloride, electrolyzing said bath while employing mercury metal as the cathode to deposit beryllium in said mercury metal and separately collecting mercury containing beryllium and thereafter separating the metal of said halide from mercury.

S. The method of producing the metal from a halide of beryllium comprising steps of incorporating said halide in a bath containing as its essential components at least one of the alkali and alkaline earth metal halides, said bath having a melting point less than the boiling point of mercury, electrolyzing said bath between an anode and a liquid mercury cathode to deposit metal in said cathode, separately collecting mercury containing said metal and heating and pressing the collected mercury to expel the mercury and to compact the metal contained therein.

6. The method of producing the metal from a halide of beryllium comprising steps of incorporating said halide in a bath containing as its essential components beryllium chloride and sodium chloride, said bath having a melting point less than the boiling point of mercury, electrolyzing said bath between an anode and a liquid mercury cathode to deposit metal in said cathode, separately collecting mercury containing said metal and heating and pressing the collected mercury to expel the mercury and to compact the metal contained therein.

7. The method of preparing finely divided beryllium metal from its halide comprising the steps of incorporating said halide in a bath containing as its essential components at least one of the alkali and alkaline earth metal halides, said bath having a melting point less than the boiling point of mercury, electrolyzing said bath between an anode and a liquid mercury metal cathode to deposit beryllium in said cathode, separately collecting mercury containing beryllium to leave finely divided beryllium metal.

8. The method of preparing finely divided beryllium metal from its halide comprising the steps of incorporating said halide in a bath containing as its essential components beryllium chloride and sodium chloride, said bath having a melting point less than the boiling point of mercury, electrolyzing said bath between an anode and a liquid mercury metal cathode to deposit beryllium in said cathode, separately collecting mercury containing beryllium and removing the mercury from said beryllium to leave finely divided beryllium metal.

References Cited in the file of this patent UNITED STATES PATENTS 1,787,672 Davenport Ian. 6, 1931 1,970,973 Palmaer Aug. 21, 1934 2,980,378 Burgess Nov. 13, 1934 2,188,904 Kjellgren et al Feb. 6, 1940 2,717,234 Nagey Sept. 6, 1955 2,774,729 Meister Dec. 18, 1956 2,796,393 Boyer June 18, 1957 2,861,030 Slatin Nov. 18, 1958 FOREIGN PATENTS 166,418 Australia Jan. 3, 1956 

1. THE METHOD OF REDUCING THE HALIDE OF BERYLLIUM COMPRISING THE STEPS OF INCORPORATING SAID HALIDE IN A EUTECTIC BATH CONTAINING AS ITS ESSENTIAL COMPONENT AT LEAST ONE OF THE ALKALI AND ALKALINE EARTH METAL HALIDES, SAID BATH HAVING A MELTING POINT LESS THAN THE BOILING POINT 